Boy, you just don't understand! Smokeless powder doesn't burn, it rearranges itself into more stable compounds that occupy a lot more space, and in the process release a lot of heat! This rearrangement is in the form of a shockwave that travels throught the powder grains, just like a high exposive does, except slower due to additives and the colloiding process. Black gunpowder does burn, but it carries its' own oxygen in it (the saltpeter/possasium nitrate).

Just how much oxygen do you think there is in a cartridge or a gun barrel? NOT ENOUGH TO SUPPORT THE COMBUSTION OF ANY POWDER CHARGE!!

A firearm will fire just fine in a vacuum as well as in the atmosphere, and underwater also!